Research Summary The recent introduction of Zika virus (ZIKV) into the New World sparked concern that the virus would emerge into a sylvatic cycle in American non-human primates (NHPs) and mosquitoes. The ability of a pathogen to emerge into a novel host species is determined, in part, by the virulence of that pathogen in the novel species. Current theory on the evolution of virulence rests on the premise that pathogen fitness is maximized by optimizing the trade-off between instantaneous pathogen transmissibility and duration of infection. While most theoretical studies of virulence evolution have focused on the trade-off between transmission and host mortality, the majority of pathogens do not kill their hosts. Instead, most infections are curtailed by the host immune response, leading to a transmission-clearance trade-off. Studies of the transmission-clearance trade-off are scarce, but we have previously found that, across multiple studies in the literature, there is an inverse relationship between peak virus titer and duration of infection when arthropod-borne viruses are experimentally inoculated into natural hosts. Moreover, we have leveraged these data to model alternate transmission strategies, namely a ?tortoise? strategy of low magnitude, long duration viremia and a ?hare? strategy of short duration, high magnitude viremia and found that arboviruses that adopted a tortoise strategy had higher rates of persistence in both host and vector populations. Nonetheless current understanding of transmission-clearance trade-offs in arboviruses is rudimentary, and integrated experimental and modeling studies of arbovirus trade-offs in ecologically-relevant host and vector species are needed to appropriately assess the risk of establishment of sylvatic cycles in new areas and the subsequent risk of emergence from such cycles. To this end, we will quantify dynamics of ZIKV and dengue virus (DENV) infection, immune response, and transmission in native NHP hosts (cynomolgus macaques) as well as novel, American NHPs (squirrel monkeys) to identify transmission-clearance trade-offs, and we will build models to predict the impact of such trade-offs on virus persistence in host populations. DENV is chosen as a counterpoint to ZIKV because, despite circulating in humans in the Americas for centuries, it has not yet established an American sylvatic cycle [17]. We will test four specific hypotheses: (i) In native hosts and novel hosts, sylvatic arboviruses experience a transmission-clearance trade-off; (ii) In native and novel hosts, the innate immune response shapes the transmission-clearance trade-off; (iii) Sylvatic arboviruses experience different transmission-clearance trade-offs in native hosts and novel hosts, resulting in less transmission from novel hosts; (iv) DENV and ZIKV lineages from human-endemic transmission cycles experience different transmission-clearance trade-offs than their sylvatic ancestors in native NHP hosts, but similar patterns in novel NHP hosts.